(a) Technical Field
The present invention relates to a method of manufacturing a 5-layer MEA having an improved electrical conductivity, which can reduce electrical contact resistance between a catalyst layer and a micro-porous layer (MPL) therein.
(b) Background Art
Extensive research aimed at using a polymer electrolyte membrane fuel cell (PEMFC) as a power source of a vehicle or a stationary power generator having a capacity of 200 kW or less has continued to progress due to its advantages such as a high output density, a high response rate, a simplified system, and the like.
In the PEMFC, a membrane-electrode assembly (MEA) is positioned at the most inner portion, in which an anode and a cathode are positioned on both sides of an electrolyte membrane.
That is, as shown in FIG. 1, catalyst layers 3, i.e., an anode and a cathode are formed by uniformly coating a desired amount of catalyst onto the surface of a polymer electrolyte membrane (PEM) 4. Gas diffusion layers (GDLs) 2 are positioned at the outside of the catalyst layers 3. Separators 1 each having a flow field for supplying fuel and exhausting water produced by a reaction are positioned at the outside of the GDLs 2.
In general, a unit cell of the PEMFC comprises one PEM, two GDLs, and two separators, and a stack cell with a desired scale can be formed by stacking the unit cells.
Referring to FIG. 2, which is a schematic diagram illustrating the transfer of reactants in the MEA having the above-described configuration, an oxidation reaction of hydrogen takes place at the anode of the fuel cell to produce hydrogen ions and electrons. The thus produced hydrogen ions and electrons are transferred to the cathode through the polymer electrolyte membrane and a conducting wire, respectively.
Simultaneously, a reduction reaction of oxygen occurs at the cathode receiving the hydrogen ions and the electrons to produce water. At this time, electrical energy is generated by the flow of the electrons through the conducting wire and by the flow of the protons through the polymer electrolyte membrane.
Conventionally, the above-described MEA is prepared by either a catalyst-coated-on-GDL (CCG) process or a catalyst-coated-on-membrane (CCM) process. In the CCG process, catalyst is coated on gas diffusion layers (GDLs) and then the catalyst-coated GDLs are combined with a polymer electrolyte membrane, producing a 5-layer MEA. In the CCM process, catalyst is coated on a polymer electrolyte membrane, producing a 3-layer MEA.
The CCG process is widely used in a laboratory scale since the manufacturing process is simple and easy. However, it is difficult to apply the CCG process in industrial fields due to its serious shortcomings in that the catalyst layer can be coated onto the GDLs only by a spray coating process and the spray coating process involves a relatively high rate of catalyst loss, thus deteriorating the overall manufacturing efficiency.
On the other hand, as a CCM process, a decal process is generally applicable to industrial fields. In the decal process, catalyst slurry is applied onto a decal (release) paper, the applied catalyst slurry is dried, followed by thermo-compression onto to polymer electrolyte membrane (refer to FIG. 3).
The decal process has an advantage in that there is hardly any resistance (proton resistance) between the electrolyte membrane and the catalyst layer since it is possible to form the catalyst layer directly onto the polymer electrolyte membrane. Nonetheless, it has a disadvantage in that contact resistance is inevitably caused in the process of stacking the gas diffusion layers (GDLs) onto the catalyst layers.
In general, the GDL includes a micro-porous layer (MPL) formed on a surface thereof to be in contact with the catalyst layer. The MPL functions to smoothly exhaust water produced by electrochemical reaction and further to facilitate physical contact between the catalyst layers having pores with diameters of several nanometers and the GDLs having pores with diameters of several microns.
The MPL includes carbon particles and a binder. The carbon particles are used to ensure electrical conductivity. The binder are used to bind carbon particles (e.g., binding between carbon particles and binding between carbon particles and the GDL) and provide waterproof performance. Teflon resin is mainly used as the binder and it can smoothly exhaust water due to its hydrophobic characteristics.
An example of a layer configuration of the conventional MEA is shown in FIG. 4. In the CCG process, as described above, the catalyst layer is coated directly on the MPL and dried. Thus, no boundary between the catalyst layer and the MPL is created and no electrical contact resistance is caused. By contrast, in the CCM process, i.e., the decal process, since the catalyst layer is in contact with the MPL by a simple connecting pressure, electrical contact resistance is caused on the boundary surface thereof.
In order to solve the contact resistance problem, various types of MPLs have been developed and commercialized. However, maximum output performance is hard to be achieved because it is difficult to accurately combine the configurations of the catalyst layer and the MPL. Moreover, providing an optimum combination thereof requires considerable amount of time and resources. Furthermore, after a new catalyst layer is developed, a new MPL suitable for the new catalyst layer still must be developed.
For example, a thermo-compression process may be used in order to remove the contact resistance between the catalyst layer and the MPL. However, Nafion used as the binder of the catalyst layer has a glass transition temperature (Tg) of about 100° C. to about 130° C. and a melting point (Tm) of about 200° C. to about 230° C., and Teflon resin used as the binder of the MPL has a Tg as high as about 340° C. Accordingly, it is impossible to connect the two layers only by the simple thermo-compression process.
Moreover, it is undesirable to use an adhesive agent since it may affect the chemical stability and cause additional contact resistance.
Accordingly, there is a need for a new manufacturing method which can overcome the above-described problems associated with prior art.
The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.